Projection display systems use high intensity lamp sources for illuminating through the image generators and optics, then through the projection optics and onto a screen. Preferred lamps for projection displays comprise a high intensity arc discharge lamp positioned within a reflective structure to produce a high intensity light beam. Particularly for digital data projectors and digital projection large screen televisions, these lamps require high temperature stable reflectors due to the small size arcs and resulting light spectrums developed to achieve more balanced color output and high brightness (lumens per square centimeter) on the screen. For portable data projectors the push for shorter arc lamps that produce higher brightness for the same has become a commercial trend.
There are many requirements for the reflector substrate material as a result of the above lamp characteristics. Due to the smaller size of the lamp the material may operate at temperatures up to about 550° C., above the service temperature of low thermal expansion borosilicate glasses. Also starting up or turning off the lamp will lead to higher transient thermal gradients in the reflector substrate creating higher thermal stresses that could lead to fracture. A very low thermal expansion coefficient (≦10×10−7 K−1) material would reduce the thermal stresses from transient thermal gradients so that thermal fracturing would be much less likely to occur.
Another requirement of the reflector substrate material is for infrared heat radiation removal or transfer through the material so less infrared heat radiation is sent along the light path. The reflective surfaces on high intensity lamp reflectors use a multilayer coating, designed to allow transmission of infrared radiation through it, while reflecting the visible radiation. It is known as a cold mirror coating. However, the reflector substrate material needs to also be transmissive to the near infrared (heat) radiation wavelengths to allow the heat removal. This requires a low level of near infrared absorbing species such as transition metal oxides, and especially iron oxides.
In order to have a highly efficient reflective surface the substrate material must have a very smooth surface texture. For a glass-ceramic material this requires a very fine grained (small crystallite size) material. An as-formed smooth surface for the reflector will negate or minimize need for polishing of the surface prior to applying the coating.
For a reasonable manufacturing process the substrate material is best formed by glass forming methods such as pressing into a mold with a precision contour plunger such that the precise contour is transferred and maintained by the glass-ceramic so that the light beam has the required geometry emitted from the lamp. Also it is desirable to lower the melting temperature of any precursor glass while still maintaining good homogeneity and low seed counts. For the needed near IR transmission, use of low iron batch materials is desired.
Various glass-ceramic materials have been proposed to make such lamp reflectors. Japanese patent publication Nos. 1992-367538 and 1992-348302 disclosed glass-ceramic material lamp reflectors having solid solutions of β-spodumene (Li2O·Al2O3·4SiO2) and/or β-eucryptite (Li2O·Al2O3·2SiO2) as the predominant crystalline phases. Glass-ceramic materials containing β-spodumene and β-eucryptite are known to be heat-resistant materials having low thermal expansion. However, it was stated in Japanese patent publication No. 1992-348302 that the crystallized product, even when started from glass precursor finished with smooth surface, usually could become rough during the crystallization process for forming the β-spodumene or β-eucryptite solid solution. The roughness of the reflector after ceramming could reach 100 mn, and sometimes could exceed 500 nm. A roughness this high is not acceptable for direct deposition of reflective coatings without further polishing of the surface.
Another problem of the prior art glass-ceramic lamp reflector is microcracking on the reflective surface. Such cracking may take place during the ceramming thermal treatment, or during the life cycle of the lamp reflector. Such cracking is normally carried over to the reflective coatings, leading to less efficient light reflection producing poor lumen output consistency.
Therefore, there remains a genuine need of heat-resistant, low thermal expansion, high surface smoothness lamp reflector substrates that can be produced without further surface polishing upon crystallization.
The present inventors have found that glass-ceramic lamp reflector substrates containing β-quartz solid solution as the predominant crystalline phase can be produced by controlling the composition of the precursor glass and the ceramming (crystallization) process. The lamp reflector substrates thus produced have the surface smoothness, heat resistance, thermal expansion and near infrared transmission required for an ultra-high pressure arc discharge lamp.
The present invention is based on this discovery.